The present invention relates to a process for separating cyclic phosphazene oligomers, and more particularly to a process for separating cyclic phosphazene oligomers from a mixture containing cyclic phosphazene oligomers and linear phosphazenes by treating the mixture with a mineral adsorbent, by which cyclic phosphazene oligomers of high purity can be readily separated in high yields.
The term "linear phosphazenes" as used herein means linear halogenophosphonitriles usually having a degree of polymerization of 2 to 50,000, and the term "cyclic phosphazene oligomers" as used herein means cyclic halogenophosphonitriles usually having a degree of polymerization of 3 to 20.
In the early part of the 19th century, it was found that hexachlorocyclotriphosphonitrile (hereinafter referred to as "trimer") was produced by the reaction of phosphorus pentachloride and ammonia or ammonium chloride, and since then, researches had been variously made to develop a use for the cyclic trimer, but the cyclic trimer had not been put into practical use till the mid-1960s. In the mid-1960s, there was reported a process for preparing a high polymer (i.e. polychlorophosphonitrile, hereinafter referred to as "phosphazene polymer") which is soluble in organic solvents by ring-opening polymerization of the cyclic trimer, and it was also found that polymeric materials having characteristics that conventional polymeric materials did not possess could be obtained by substituting various nucleophilic substituent groups for chlorine atom bonding to phosphorus atom constituting the skeleton of the phosphazene polymer molecule.
Because of the properties as inorganic material, there have been extensively made researches for applications of the phosphazene polymers to the industrial and medical fields such as fireproofing foamed rubbers, flame retarders for plastics, O-rings, gaskets, hoses for hydrocarbon fuels, substitute blood vessels and artificial internal organs. With the increase of the use of the phosphazene polymers, the demand for cyclic phosphazene oligomers, particularly the cyclic trimer has increased, and on the other side, there have been proposed various processes for the preparation of the cyclic phosphazene oligomers.
The reaction of phosphorus pentachloride or an organic group-substituted phosphorus chloride with ammonia or ammonium chloride is a popular process for the preparation of cyclic phosphazene oligomers (PNCl.sub.2).sub.n wherein n is an integer of 3 or more. For instance, the cyclic trimer is generally prepared as follows: Phosphorus pentachloride and a slight excess of ammonium chloride are reacted in a halogenated organic solvent such as symtetrachloroethane or chlorobenzene in the absence or presence of a catalyst under reflux. Since linear phosphazenes are necessarily by-produced, after thoroughly releasing the generated hydrochloric acid from the reaction system cyclic phosphazene oligomers are then separated from the linear phosphazenes by distilling away the solvent and subjecting the residue to extraction with a solvent capable of dissolving the cyclic phosphazene oligomers but not dissolving the linear phosphazenes, e.g. petroleum ether or hexane. The cyclic trimer is recovered from the obtained cyclic phosphazene oligomers by a post-treatment such as a distillation, sublimation or recrystallization method.
However, there are some problems in such a preparation of cyclic phosphazene oligomers. As stated above, the by-production of the linear phosphazenes is unavoidable, and it is necessary to separate the cyclic phosphazene oligomers from the linear phosphazenes. However, since there is no appropriate extraction solvent which is incompatible with reaction solvents, the separation must be conducted by troublesome distillation-extraction procedures. Moreover, in order to conduct complete separation, the reaction solvent must be completely distilled away, and upon the distillation the cyclic trimer and octachlorocyclotetraphosphonitrile (hereinafter referred to as "tetramer") are liable to be lost. Also, a large quantity of an extraction solvent is required to conduct satisfactory extraction. Even if such a troublesome separation procedure is conducted, complete extraction is impossible and the extraction ratio of the cyclic phosphazene oligomers from the reaction product is at most 90% by weight. Also, some incorporation of linear phosphazenes into the extract is unavoidable. It is general practice to recover the cyclic trimer from the extracted cyclic phosphazene oligomers by a distillation, sublimation or recrystallization method, but the incorporation of the linear phosphazenes, even in a trace amount, tends to cause gelation of the cyclic phosphazene oligomers during the above recovering procedure such as distillation and the yield of the cyclic trimer is remarkably decreased.
When a catalyst, e.g. a metal or a metal salt is employed in the reaction, a part of the metal or metal salt catalyst may form adducts with linear phosphazenes and also with cyclic phosphazene oligomers, especially with the cyclic phosphazene oligomers of more than decachlorocyclopentaphosphonitrile (hereinafter referred to as "pentamer"), and these adducts often cause gelation upon recovering the cyclic trimer. Since the adducts cannot be separated by the extraction separation, this is also a problem to be solved.
A wet process using a solvent and a dry process using no solvent are known as the process for preparing cyclic phosphazene oligomers. Since the former can usually produce the cyclic phosphazene oligomers in higher yields than the latter, attempts for industrialization have been made in general by a wet process. When the recovered solvent is reused, however, the yields of the cyclic phosphazene oligomers are remarkably lowered. Therefore, purification of the recovered solvent is necessary and this increases the production cost.